The present invention relates to a method for the purification of a rare earth oxysulfide. More particularly, the invention relates to a method for the purification of a rare earth oxysulfide by removing sulfur contained therein so as to give a rare earth oxysulfide product having greatly increased whiteness to be suitable, for example, as a material of the scintillators for X-ray CT.
Rare earth oxysulfides used as a material of X-ray CT scintillators are usually prepared by the reaction of hydrogen sulfide with a rare earth oxide at a high temperature. A problem in the rare earth oxysulfide as produced by this reaction is that the oxysulfide thus produced is slightly colored in grey, presumably, due to the content of sulfur therein in an amount of 1% by weight or even larger. Such a colored rare earth oxysulfide is undesirable as a material of scintillators in respect of the decrease in the light transmissivity. Accordingly, it is eagerly desired to develop a method for obtaining a pure white product of a rare earth oxysulfide by removing the sulfur impurity but none of the prior art methods can provide a complete solution of the problem.